Background of the Art
Initially, resists were applied to substrates in liquid form. The resist was imagewise subjected to radiation to form relatively soluble and relatively insoluble areas. Upon treatment with an appropriate developer solution, the relatively soluble areas were removed and a physical mask in the desired image pattern was left on the surface of the substrate. The exposed area of the substrate would then be treated by either deposition of material onto the exposed areas (e.g., metal deposition by vapor coating or electrolytic processes) or etching of the surface of the substrate in the area exposed through the mask.
One of the first advances in the use of dry-film photoresist materials was discussed in U.S. Pat. No. 3,469,982 in which a laminable negative-acting single layer film was adhered to a substrate and used as a photoresist.
U.S. Pat. No. 4,193,797 discloses the use of single layer laminable films of positive-acting dry-film photoresist compositions for use in photoresist processes.
Improved dry-film photoresist compositions are disclosed in U.S. Pat. No. 4,247,616. This composition provides good light sensitivity, stability, and excellent resist characteristics.
U.S. Pat. No. 4,349,620 discloses a multi-layer photosensitive film resist (positive- or negative-acting) having a plurality of layers with differing adhesive properties. In particular, a carrier layer having a first photosensitive resist composition thereon and then a second photoresist with different adhesive properties on the first photosensitive composition is disclosed.
Other developments in the art have lead to the use of two layer or multilayer laminable or coatings of photoresist materials in order to improve the sensitometric or physical properties of the dry-film resist. U.K. Pat. No. 1,493,833 shows the use of a photoresist comprising a carrier layer, a photosensitive layer (including positive-acting photosensitive layers) and a non-photosensitive layer which is soluble in aqueous or organic solvents. A host of natural and synthetic polymers are disclosed for these non-photosensitive layers. U.S. Pat. No. 4,204,009 discloses the use of a resist having two photosensitive layers of different reactivities adjacent each other on top of the substrate to be treated. U.S. Pat. No. 4,217,407 discloses the use of multiple layer resist materials which comprise an o-quinone diazide containing layer in combination with at least one other layer which is permeable, swellable, or soluble by alkaline envelopers. A brief list of non-photosensitive compositions is provided.
U.S. Pat. No. 4,191,573 shows a photosensitive image element comprising the support to be plated or etched with two independently applied liquid resists of photosolubilizable layers. The resist layers comprise an azide compound and a polyamide.
U.S. Pat. No. 4,180,604 discloses the use of two separately coated liquid photosensitive resist layers. The compositions appear to be the same, with larger amounts of solvent diluent used in coating the second layer. The lower layer is more readily soluble, promoting undercutting in the formation of the resist image.
U.S. patent application Ser. No. 428,475, filed on Sept. 29, 1982, in the name of P. M. Koelsch and J. P. Vikesland discloses an improved two layer photoresist construction in which one layer is a photosensitive, positive-acting resist composition and the other layer bonding the composite to the surface of the article to be etched or plated is a crosslinked, light-insensitive thermally laminable adhesive layer. The crosslinking provides for reduced undercutting of the resist image.